1. Field of the Invention
The present invention relates to a narrow directional microphone including an acoustic tube incorporating a microphone unit. More specifically, the present invention relates to a harrow directional microphone that allows adjustment to a desired narrow directivity with low cost and without cumbersome processes.
2. Description of the Related Art
Narrow directional microphones are known that include an elongated acoustic tube. In such narrow directional microphones, a microphone unit is provided in an inner space of one end of an acoustic tube, and an acoustic wave entering the acoustic tube from an opening at a front end, that is, the other end of the acoustic tube, is detected and converted into an acoustic signal by the microphone unit. For example, narrow directional microphone that is an acoustic tube is known in which: the acoustic tube is made of metal; an opening, e.g., a slit, arranged along the center axis is formed on a peripheral wall of the acoustic tube; and the opening is covered with an acoustic resistor such as a thin synthetic resin film or a non-woven fabric attached on the peripheral wall of the acoustic tube. A narrow directional microphone having such a structure assures narrow directivity by making acoustic waves enter the acoustic tube from the front opening interfere with an acoustic wave entering the acoustic tube through the opening on the peripheral wail via the acoustic resistor.
An example of a conventional narrow directional microphone using an acoustic tube is schematically described below. In FIGS. 8 and 9, a narrow directional microphone 19 includes: an acoustic tube 12; an acoustic resistor 14; a microphone unit 18; and a grip 17. The acoustic tube 12 is an elongated cylinder made of metal and the like and has a plurality of slit 13 on a peripheral wall thereof. The slits 13 collinearly run along the center axis of the acoustic tube 12. The length of the slits 13 in the direction of the center axis becomes shorter in a stepwise manner as the slit 13 get closer to a portion at which the microphone unit 18 is provided. Therefore, acoustic resistance of each of the slits 13 becomes higher as the slits 13 get closer to the portion at which the microphone unit 18 is provided. Thus, narrow directional microphone which has the flat and excellent frequency characteristic can be obtained. If the lower and the upper end of the acoustic tube 14 as shown in FIG. 8 are the front and the rear end of the acoustic tube 14, the microphone unit 18 is provided in the inner space at the rear end. On an outer peripheral wall of the acoustic tube 12, the acoustic resistor 14 covering the slits 13 is attached. The acoustic tube 12 has both ends in the longitudinal direction opened. A front opening of the acoustic tube 12 guides in an acoustic wave from the front. The cylindrical grip 17 is integrally and continuously connected to the acoustic tube 12 at the rear end thereof.
Japanese Patent Application Publication No. S62-118697 discloses an invention related to a narrow directional microphone. In the narrow directional microphone according to the invention disclosed in Japanese Patent Application Publication No. S62-118697, an acoustic tube has openings provided on a plurality of portions in a circumferential direction thereof; and a plurality openings providing acoustic resistance and arranged in an axial direction with intervals therebetween. The acoustic tube also has a diaphragm attached on an interference pipe and covers the openings. By covering the openings with the diaphragm, an acoustic wave guided into the acoustic tube from the front is made to interfere with an acoustic wave with the phase largely varied. Therefore, acoustic waves other than that from a sound source at the front of the acoustic tube are largely attenuated. Thus, extremely narrow directivity can be obtained.
Unfortunately, trial production or processing of an opening of such acoustic tube requires certain man-hour costs. In other words, cumbersome processes have to be carried out on an acoustic tube made of metal each time to obtain appropriate narrow directivity. In addition, because processing of the acoustic tube requires high accuracy, the yield rate is degraded and the manufacturing cost becomes high.